1. Technical Field
The present invention relates to a method of data transfer in a mobile or a fixed (static) telecommunications system. The invention is particularly suitable for use in a system comprising a heterogeneous mix of mobile and fixed end devices or clients having different connection resources and requirements.
2. Related Art
Protocols developed initially for Internet connection, such as the well known Transmission Control Protocol (TCP), which controls data transfer, and the Internet Protocol (IP), which controls routing, are now largely platform independent and provide robust connections between clients and servers, for example. However, TCP/IP and the associated User Datagram Protocol (UDP) were developed when access to the Internet was almost exclusively from and to fixed nodes.
The advent of laptops, mobile (cellular) telephones, hand-held devices (PDAs) and other portable devices for which Internet connection is possible has caused these traditional Protocols to be reconsidered, however.
For example, the Internet Protocol assumes that the end point connection is at a fixed location and thus the IP address, of the form AAA.BBB.CCC.DDD, is bound to the end point computer identifier. The problem with this is that, if the computer moves but the IP address stays the same, there is no information available to the data sender indicating where the computer has moved to. Thus, the IP address must be updated every time a computer moves which results in large delays in data transfer whilst the computer is on the move.
There are a number of further fundamental differences in the manner of operation of mobile and fixed devices. For example, mobile devices tend to have limited power resources (often, a rechargeable battery) which militate against permanent connection and/or high transmission power. Also, wireless transmission and reception of data tends to be more susceptible to interruptions and drop-outs than wired connections, particularly as the mobile device moves around. It is thus particularly desirable that mobile Internet connections are optimised for graceful disconnection when data transmission is interrupted, coupled with rapid and seamless reconnection as soon as it is possible. Between such times, the hibernation should be as efficient as possible.
These issues are discussed in “Reconsidering Internet Mobility” by Snoeren et al, in Proc. 8th Workshop on Hot Topics in operating Systems (HOTOS-VIII).
A large number of approaches to the wireless connection of mobile devices to the Internet have been proposed. For example, the Mobile Internet Protocol (Mobile IP), outlined in Request for Comments (RFC) 2002, suggests the use of a ‘home agent’ which permits roaming of the mobile device by identifying a static home address regardless of the actual point of connection of the mobile device. Data is tunnelled to and from the mobile device by packet forwarding via a home link to the home agent. This arrangement is transparent to layers in the TCP/IP stack above IP; domain name service (DNS) entries refer to a home address and are not concerned with the actual Internet access point.
Mobile IP version 6, defined in various IETF drafts, suggests a number of improvements to the Mobile Internet Protocol, such as the use of a care-of address which is a temporary address that identifies the current, non-home location of the mobile device's Internet connection node (to reduce triangular routing problems, for example). The problems with mobile IP version 6 include the management at the network level of the use of the home agent which tends to clog up the home network with unnecessary traffic. The need for predictability requires part of the already limited bandwidth resources to be reserved. The tunnelling of all traffic along the tunnel link prevents selection of different or most optimized routes for different data types. Session mobility between different mobile devices is not supported.
Another exemplary approach to address the demands of mobile Internet connections is set out in “An end-to-end Approach to Host Mobility”: 6th ACM/IEEE Int. Conf. on Mobile Computing and Networking, 2000. Here, secure dynamic updates to the DNS are employed to track the location of the mobile device so that control over mobility is handed to the mobile device itself. A new set of TCP connection migration protocols are also defined. These include the use of a token which identifies a previously established connection to an address, port pair. The devices at either end of the connection thus need to negotiate to agree upon a value for the identifying token. A previously started TCP connection needs to be restarted when an address change occurs in this technique.
Still a further approach employs Session Layer Mobility Management (SLM) which moves the management of the mobility of the mobile device from the base station or home agent onto the mobile device itself. SLM. operates above TCP and switches TCP streams between connections. SLM has two parts; a session management part which resides on the mobile device and manages dynamic connection during a session, and a location management part which resides within the communications network and assists in device location within the network.
The use of conventional dynamic host configuration protocol (DHCP) to assign IP addresses to mobile devices as they move between base stations requires that any mobile device attached to the network must be IP networking enabled. This places limits on the diversity of devices that can be attached to the network. The techniques outlined in the preceding two documents are likewise constrained by the need for the mobile or other end device to be IP enabled.
“Advocating a remote socket architecture for Internet access using Wireless LANs”, Special Issue of MONET on Wireless Internet and Intranet Access (1999) advocates an approach which employs a remote socket architecture. A socket, as will be understood by those skilled in the art, acts to strip out data received in TCP packets and forwards the data to an application. In the other direction, the socket acts to bundle data in a socket buffer into TCP packets. In this document, the proposed architecture locates a socket interface and the TCP/IP stack on a base station of a wireless network, with a user interface on the mobile device. Applications on the mobile device are routed via a local socket module using a Last Hop Protocol.
The base station thus acts as a proxy for the mobile device.
Whilst the architecture described in this document allows communication with or between non IP enabled devices, it does have a number of drawbacks. Only stationary wireless access is considered, so that the difficult problems of mobile device and/or session mobility are not addressed. Moreover, the primary goal of this architecture is the improvement of TCP performance over a wireless link.